The use of X-rays is a widespread method for depicting the internal structure of objects, in particular in medical imaging or in non-destructive material testing. Herein, the object to be depicted is often very heterogeneous, i.e. there are regions of the object in which X-rays are absorbed weakly (in medical imaging, lung tissue for example) and simultaneously regions in which radiation is strongly absorbed (in medical imaging, bone regions for example). Therefore, depictions of the object have high dynamics in the registered X-ray intensities.
A method is known from the patent DE 10 2010 041 176 B4 for operating X-ray units with automatic lighting control. Herein, the aim of automatic lighting control is to achieve good image quality in the relevant regions of the image and simultaneously the lowest possible X-ray dose absorption by the object. Automatic lighting control is in particular used in fields of application in which multiple X-ray images of the same object are recorded, for example in fluoroscopy or the X-ray angiography. Herein, fluoroscopy (an equivalent term is “transillumination”) is understood to mean the continuous recording of processes in an object, in particular in a human body, by way of X-rays. X-ray angiography is a typically time-resolved depiction of vessels by way of X-rays, wherein contrast medium is injected into the vessels.
The X-rays generated in known X-ray units are homogeneous, i.e. both strongly and weakly absorbent regions of the object to be depicted are irradiated with the same X-ray intensity. This represents an additional challenge for the lighting control.
However, the X-ray intensity registered in the X-ray detector is not the only factor influencing image quality. The signal-noise ratio, and hence the image quality, furthermore depend upon the content of scattered radiation. Scattered radiation generally has much lower X-ray dynamics than primary radiation. Therefore, regions with a strong X-ray absorption have a much higher content of scattered radiation than regions with low X-ray absorption.